Self-driving cars that can steer and brake themselves risk lulling people in the driver’s seat into a false sense of security—and even to sleep, reports The Associated Press.

Among the 48 students put in the driver’s seat, 13 who were instructed to monitor the car and road began to nod off. Only three did so when told to watch a video or read from a tablet.

Alertness mattered when students needed to grab the wheel because a simulated car or pedestrian got in the way.

There’s no consensus on the right car-to-driver handoff approach: the Stanford research suggests engaging people with media could help, while some automakers are marketing vehicles with limited self-driving features that will slow the car if they detect a person has stopped paying attention to the road.